Numerous autopilot, autothrottle, and flight guidance systems for use in aircraft flight control have been developed in the prior art. Such systems have often evolved in a piecemeal fashion. Particularly with respect to longitudinal axis flight control, such automatic control systems are characterized by a proliferation of control laws and hardware components. The same is true for computer-augmented manual control systems (often referred to as fly-by-wire control systems.) As a result, these systems are overly complex and lacking in functional integration. This has caused numerous operational and performance drawbacks.
In response to this situation, a fully integrated vertical flight path and speed control system was developed and is described in U.S. Pat. No. 4,536,843, incorporated herein by reference. This system is termed a Total Energy Control System (hereinafter referred to as "TECS" or "TEC system".) The TEC system develops fundamental solutions to the problem of coordinated elevator and throttle control to produce performance levels exceeding those generally known in the flight control system's art. TECS provides functionally integrated control for all autopilot and flight management system modes, as well as for computer-augmented manual control modes.
The basic design philosophy of a TEC system is to compute an aircraft's total energy state and its desired energy state, as represented by flight path, speed and their associated targets. The TEC system controls the total energy error with thrust, while using the elevator to control the energy distribution error between the flight path and speed. For all flight conditions, thrust is the most effective means to change the aircraft's energy state, whereas elevator control provides an effective means to modulate energy distribution and stabilize the aircraft's attitude.
In more detail, should a pilot want to change from a present flight path angle .gamma. to a commanded flight path angle .gamma..sub.C and/or to change the longitudinal acceleration from a present value V to a commanded value V.sub.C, the engine throttles may be driven until the total specific energy rate error, E.sub.S.epsilon., relative to the combined target flight path angle and longitudinal acceleration is zero. ##EQU1## where, EQU V.sub..epsilon. =V.sub.C -V (2) EQU .gamma..sub..epsilon. =.gamma..sub.C -.gamma. (3)
The signal V.sub..epsilon. /g is the longitudinal acceleration error signal normalized by the gravitational acceleration constant g.
Likewise, the elevator surfaces are re-positioned until the energy rate distribution error, D.sub..epsilon., representing the difference between the flight path angle error and the longitudinal acceleration error, is zero. ##EQU2##
The TECS control concept has been shown to work effectively in various speed and flight path control modes for a multitude of flight conditions.
FIG. 1 shows the general functional architecture of the Total Energy Control System. The Flight Control Computer (FCC) depicts the major subfunctions of the TECS processing algorithm. The FCC receives data inputs from an Inertial Reference Unit (IRU), an Air Data Computer (ADC), a Mode Control Panel (MCP), and a Flight Management Computer (FMC). The ADC and IRU provide sensor information on the airplane states and dynamics, such as airspeed, altitude, vehicle accelerations, angular rates, attitudes, and aircraft latitude and longitude position. The MCP provides engagement and disengagement control of the various automatic and computer augmented manual control modes, as well as means to select and display the control reference commands for the tactical automatic flight control modes. The MCP is generally mounted in the center of the cockpit glareshield. The FMC provides the control reference commands for strategic (preprogrammed) flight control modes, often called VNAV (for vertical navigation), LNAV (for longitudinal navigation), and VPROF (for speed profile). For these strategic modes, the vertical and horizontal flight paths and speed profile are programmed as a function of aircraft latitude and longtitude, or in the case of the speed profile mode, as a funtion of altitude in the FMC via the Control Display Unit (CDU).
The control algorithm in the FCC is subdivided into the following main parts, shown as blocks labeled Path Modes Feedback Normalization, Speed Modes Feedback Normalization, Commands Coordination, and Control Column Command Processing. In the Path Modes Feedback Normalization block, the path mode error for any selected vertical flight path control mode is normalized into a flight path angle command signal .gamma..sub.C which is passed on to the block labeled Commands Coordination. Likewise, in the Speed Modes Feedback Normalization block, a speed error for the selected speed mode is normalized to form the longitudianl acceleration command.
In the Commands Coordination block, the energy rate related flight path angle and longitudinal acceleration commands are processed into a specific net thrust command and pitch innerloop control command. The Control Column Command Processing block receives a signal that is sensed from pilot movement of the control column or stick. The Control Column Command Processing block processes this signal to form the commands which are passed on to the Commands Coordination block to shape the airplane flight path angle and pitch attitude responses for the computer augmented manual mode (sometimes referred to as a "fly-by-wire" mode) to provide optimum handling qualities.
The block Thrust Scaling scales the specific thrust command to the actual aircraft weight and the number of operating engines to form the individual Net Thrust Commands to each engine. The block Pitch Innerloop provides the short period pitch attitude stabilization for the aircraft. The engine thrust may be controlled manually by the pilot using the throttles, or automatically by the FCC.
Referring to FIG. 2, block 38 illustrates the basic circuitry for realizing the above-described TECS control concept. Commanded flight path angle signal .gamma..sub.C is applied as one input to a combiner 12. The aircraft's actual or current flight path angle signal .gamma. is also inputted to combiner 12. The combiner 12 outputs a flight path error signal .gamma..sub..epsilon. equal to the difference between the commanded and actual flight path angles.
Similarly, the aircraft's actual or current longitudinal acceleration signal V is subtracted from the commanded longitudinal acceleration V.sub.C in a combiner 14. The resulting combiner 14 output is the longitudinal acceleration error signal V.sub..epsilon.. An amplifier 16 multiplies the V.sub..epsilon. error by a fixed gain ##EQU3## producing a non-dimensionalized output signal V.sub..epsilon. /g which is provided as inputs to combiners 18 and 20. Also coupled as inputs to combiners 18 and 20 is the signal .gamma..sub..epsilon.. The output signal from combiner 20 is the addition of these quantities to form E.sub.S.epsilon. =.gamma..sub..epsilon. +V.sub..epsilon. /g, which is the above-stated total energy rate error. The output signal from combiner 18 is the difference of these quantities to form D.sub..epsilon. =-.gamma..sub..epsilon. +V.sub..epsilon. /g, which is the above-stated energy rate distribution error.
The specific energy rate error signal E.sub.S.epsilon. is applied to thrust command computation circuitry 22 which generates a thrust command signal .delta..sub.THRUST.sbsb.C. The thrust command signal .delta..sub.THRUST.sbsb.C is used to control engine thrust at block 24, and is calculated to reduce the specific energy rate error signal E.sub.S.epsilon. to zero. In a similar manner, the energy rate distribution error signal D.sub..epsilon. is fed to elevator command computation circuitry 26, which responds by producing an elevator control command signal .delta..sub.ELEVATOR.sbsb.C. This signal, when applied to the aircraft's elevator at block 24, works to drive the energy rate distribution error signal D.sub..epsilon. to zero in harmony with the control of E.sub.S.epsilon.. In the above manner, the aircraft is precisely guided from its present flight path angle and longitudinal acceleration to the flight path angle and longitudinal acceleration having the least amount of energy error and distribution error.
Frequently, it is desired to control the aircraft to altitude and speed targets rather than to flight path angle and longitudinal acceleration targets. In that case, a simple process may be used to normalize the command and feedback signals of the selected flight path and speed mode into the standard .gamma..sub.C and V.sub.C signals. (This normalization is shown in FIG. 3.) The air speed error V.sub..epsilon. is multiplied by a suitable gain K.sub.V to form the longitudinal acceleration command: EQU V.sub.C =K.sub.V V.sub..epsilon. (5)
where, EQU V.sub..epsilon. =V.sub.C -V (6) EQU V.sub..epsilon. =V.sub.C -V (7)
with V being the current airspeed, V.sub.C being the commanded airspeed, V being the current longitudinal acceleration, and V.sub.C being the commanded longitudinal acceleration.
The altitude error is multiplied by a suitable gain factor K.sub.h to form the vertical speed command h.sub.C =K.sub.h h.sub..epsilon.. The gain K.sub.h is selected equal to K.sub.V to yield identical altitude and speed dynamics. Subsequently, the vertical speed command is divided by the aircraft's speed V to form the flight path angle command: ##EQU4## and ##EQU5##
The altitude and speed errors are thus scaled in correct relative energy terms. Given the above signal normalization, the flight path angle and longitudinal acceleration commands can be developed for each of the standard longitudinal autopilot and autothrottle modes to couple into the generalized total energy based thrust and elevator command processors.
Referrring back to FIG. 2, the computer-augmented manual control mode of known TECS is provided by processing the signal .delta..sub.C representative of the pilot's control column or sidestick input to form the flight path angle command signal .gamma..sub.C.sbsb.MAN. The term .gamma..sub.C.sbsb.MAN is shown as an input to combiner 12 in FIG. 2. Conceptually, the pilot's input processing module for the computer-augmented manual control mode is shown in FIG. 1 and labeled "command processing." This command processing module also produces other signals that are used to precisely shape the control responses to achieve optimum handling qualities throughout the flight envelope.
The above-described TECS system, however, does not yet include all the desired functionality. One such function has to do with envelope safeguarding and speed stability when thrust is limited to its upper or lower limit.
As background information, conventional unaugmented airplane controls do not typically include explicit envelope safeguarding function, such as angle of attack limiting. Therefore, full column push or pull can result in overspeed or stall conditions. That is why regulatory authorities include regulations explicitly written to ensure adequate safety in aircraft design. United States regulations require the presence of a natural or synthetic stall warning device and a demonstration that the airplane is, in fact, speed stable. Speed stability means that when the airplane is trimmed for a certain speed and the airplane is subsequently maneuvered (pitched) at constant thrust in a manner that causes a departure from the trim speed, the airplane will naturally return to its trim speed after the column (or applicable pitch controller) is released.
For computer-augmented control systems that have neutral or negative static speed stability, the above regulatory requirement is met by a demonstration of "equivalent" safety, achieved by the addition of mode functions intended to limit angle of attack to a particular value or to limit speed to a minimum value V.sub.MIN, as well as to a maximum value V.sub.MAX. These limits are shown in FIG. 2 at box 34 and are used to override the speed and energy distribution signals at switches 32 and 36, respectively.
The issue of speed stability arises in the TECS context as follows. As stated above, the TEC system controls the total energy error with thrust, while the energy distribution error is controlled with the elevator. While in the fully automatic modes (i.e., Flight Management System, or autopilot with autothrottle) using the basic TEC system, the pilot selects a speed mode and associated speed target, as well as a flight path mode and an associated flight path target. The selection of target speed and target flight path can cause the thrust to increase (or decrease) to its maximum thrust limit (or idle thrust limit.) When this happens, there is only one controller left (the elevator) and therefore only one of the control targets can then be satisfied directly. For such conditions, which are usually of short duration, a priority selection must be made as to which control target should be satisfied first--flight path or speed. This priority selection is called "control priority" and for many automatic mode combinations, it is most appropriate to select a "speed control priority."
Referring to FIG. 2, speed control priority by use of the elevator only is achieved by opening switch 28. This action disconnects the flight path angle error signal .gamma..sub..epsilon. from the elevator command computation so it will not interfere with attaining or maintaining the target airspeed. This allows the TEC system to continue using the elevator to control to a particular airspeed target, while the flight path angle is temporarily left to respond in an open-loop fashion.
For some modes (e.g., the computer-augmented manual control mode and the ILS Glide Slope mode), a flight path control priority is used when the command thrust reaches its upper or lower limit. In this case, the basic TEC system opens a switch, labeled 30 in FIG. 2, to eliminate the acceleration error term from the elevator command computation. This allows the TEC system to continue using the elevator to control to a particular target flight path, while temporarily letting the speed respond in an open-loop fashion.
Relating the above situations to the TECS computer-augmented manual control mode, the TEC system will control to both flight path and speed targets as long as the thrust command stays within the minimum and maximum thrust limits. In that case, there is no speed stability issue because speed will be maintained automatically. Should the thrust command reach a limit, then the switch 30 is opened and the airspeed is allowed to decrease (or increase) until the flight path angle is satisfied or a speed limit is reached, whichever occurs first. Thus, for the TECS computer-augmented manual control mode, provides full freedom to manually control the flight path angle and will only constrain the flight path control to ensure the selected speed limits are not exceeded, after thrust has reached a command limit.
The above-described TEC system provides safety with regard to speed that is at least as good as that provided by classical airplanes having positive speed stability in combination with some type of stall warning device. The above-described TEC system also produces satisfactory handling qualities for most flight conditions.
There is, however, one flight condition in which this control strategy may not produce optimum handling qualities. For example, during manual mode flight after takeoff, it is usual for the pilot to want to establish the aircraft's a flight path angle consistent with the set limit thrust, while establishing and maintaining a target climb-out speed. For this case, path control priority allows good control over flight path, but it may be difficult for the pilot to establish the exact flight path angle command consistent with the target climb-out speed and, if established, to maintain that speed, since for this condition speed is not being directly controlled. This is particularly the case if the airspeed has a tendency to diverge. The result is that the pilot will need to manipulate the flight path angle target more frequently than desired in order to indirectly control to the target speed.
The problem may be better understood considering the following. After establishing the correct inertial flight path with thrust at the upper limit and speed at the target speed, the speed may subsequently start to drift off due the aforementioned lack of speed stability, or due to the effect of a wind change, which at constant inertial flight path angle, will change the flight path angle with respect to the air mass and thus will affect the longitudinal acceleration. Also, the change in net thrust due to change in altitude may cause the pilot to repeatedly change his flight path angle command in order to maintain speed at the target value. This is both difficult and workload intensive.
Thus, when flying in the computer-augmented manual control mode of the TEC system, a need exists for reduction of the pilot workload during thrust-limited conditions or auto thrust disengaged conditions in a TEC system, especially during the task of establishing and maintaining a speed target during climb-out at constant thrust. The present invention is directed to fulfilling this need.